diff --git a/docs/api/utils/combineKeys.md b/docs/api/utils/combineKeys.md
index 7b9f49e..0fb32b0 100644
--- a/docs/api/utils/combineKeys.md
+++ b/docs/api/utils/combineKeys.md
@@ -8,7 +8,7 @@ of each key.
 
 ```ts
 export const combineKeys = <K, T>(
-  keys$: Observable<Array<K> | Set<K>>,
+  keys$: Observable<Iterable<K> | KeyChanges<K>>,
   getInner$: (key: K) => Observable<T>,
 ): Observable<Map<K, T>>
 ```
@@ -53,8 +53,8 @@ const petRace$ = petUpdate$.pipe(startWith(
   ...petNames.map((pet, id): Pet => ({pet, id, pos: 1})),
 ));
 
-const [petByID, petIds$] = partitionByKey(petRace$, x => x.id)
-const keyMap$ = combineKeys(petIds$, petByID);
+const [petByID, petIdChange$] = partitionByKey(petRace$, x => x.id)
+const keyMap$ = combineKeys(petIdChange$, petByID);
 
 const leadingPet$ = keyMap$.pipe(map(x => // map to pet with highest pos
   Array.from(x.entries())
@@ -76,7 +76,6 @@ const advancingPet$: Observable<Pet> = interval(1000).pipe(
   tap(updatePet),
 );
 
-const [usePetIDs] = bind(petIds$);
 const [usePetByID] = bind((petId:number) => petByID(petId));
 const [useLeader] = bind(leadingPet$, null);
 const [useAdvancingPet] = bind(advancingPet$, null);
diff --git a/docs/api/utils/partitionByKey.md b/docs/api/utils/partitionByKey.md
index 894245c..9952b57 100644
--- a/docs/api/utils/partitionByKey.md
+++ b/docs/api/utils/partitionByKey.md
@@ -11,7 +11,12 @@ export function partitionByKey<T, K, R>(
   stream: Observable<T>,
   keySelector: (value: T) => K,
   streamSelector: (grouped: Observable<T>, key: K) => Observable<R>,
-): [(key: K) => GroupedObservable<K, R>, Observable<K[]>]
+): [(key: K) => GroupedObservable<K, R>, Observable<KeyChanges<K>>]
+
+interface KeyChanges<K> {
+  type: "add" | "remove"
+  keys: Iterable<K>
+}
 ```
 
 #### Arguments
@@ -26,13 +31,13 @@ export function partitionByKey<T, K, R>(
 
 1. A function that accepts a key and returns a stream for the group of that key.
 
-2. A stream with the list of active keys
+2. A stream of KeyChanges, an object the describes what keys have been added or deleted.
 
 ### Examples
 
 ```ts
 const source = interval(1000);
-const [getGroupByKey, keys$] = partitionByKey(
+const [getGroupByKey, keyChanges$] = partitionByKey(
   source,
   x => x % 2 == 0 ? "even" : "odd",
   (groupedObservable$, key) => groupedObservable$.pipe(map(x => `${x} is ${key}`))
@@ -40,7 +45,12 @@ const [getGroupByKey, keys$] = partitionByKey(
 
 const [useEven, even$] = bind(getGroupByKey("even"));
 const [useOdd, odd$] = bind(getGroupByKey("odd"));
-const [useKeys] = bind(keys$);
+const [useKeys] = bind(
+  keys$.pipe(
+    toKeySet(),
+    map(keySet => Array.from(keySet))
+  )
+);
 
 function MyComponent() {
   const odd = useOdd();
@@ -76,13 +86,16 @@ const [petUpdate$, updatePet] = createSignal<Pet>();
 // Let's line up our pets
 const petRace$ = merge(of(...petNames), petUpdate$);
 
-const [petByID, petIds$] = partitionByKey(
+const [petByID, petIdChange$] = partitionByKey(
   petRace$,
   x => x.id,
 )
 
 const [usePetByID] = bind((id: number) => petByID(id));
-const [usePetIDs] = bind(petIds$);
+const [usePetIDs] = bind(petIdChange$.pipe(
+  toKeySet(),
+  map(keySet => Array.from(keySet))
+));
 
 const PetItem = ({petID}: {petID: number}) => {
   const pet = usePetByID(petID);
@@ -111,3 +124,4 @@ const PetList = () => {
 ## See also
 
 - [`combineKeys()`](combineKeys)
+- [`toKeySet()`](toKeySet)
diff --git a/docs/api/utils/toKeySet.md b/docs/api/utils/toKeySet.md
new file mode 100644
index 0000000..bcbcc1d
--- /dev/null
+++ b/docs/api/utils/toKeySet.md
@@ -0,0 +1,64 @@
+---
+title: toKeySet()
+sidebar_label: toKeySet()
+---
+
+Operator function that maps a stream of KeyChanges into a Set that contains the
+active keys.
+
+```ts
+function toKeySet<K>(): OperatorFunction<KeyChanges<K>, Set<K>>
+```
+
+#### Returns
+
+An [OperatorFunction] that will aggregate the key changes from the stream into
+a Set with those keys.
+
+### Example
+
+```ts
+// Call to service mocked for the example
+const priceStream$ = from([
+  {
+    symbol: "AUDCHF",
+    price: 1.32,
+  },
+  {
+    symbol: "GOLD",
+    price: 1.213,
+  },
+  {
+    symbol: "CRYPTO",
+    price: 2.45,
+  },
+  {
+    symbol: "GOLD",
+    price: 1.27,
+  },
+]).pipe(
+  // Add delay to every new price.
+  concatMap((value) => of(value).pipe(delay(100))),
+)
+
+const [lastPriceOfSymbol, symbolChange$] = partitionByKey(
+  priceStream$,
+  (x) => x.symbol,
+)
+
+const activeSymbols$ = toKeySet(symbolChange$).pipe(
+  map((keySet) => Array.from(keySet)),
+)
+
+activeSymbols$.subscribe((keys) => console.log(keys))
+// Will log:
+// ['AUDCHF']
+// ['AUDCHF', 'GOLD']
+// ['AUDCHF', 'GOLD', 'CRYPTO']
+```
+
+## See also
+
+- [`partitionByKey()`](partitionByKey)
+
+[operatorfunction]: https://rxjs.dev/api/index/interface/OperatorFunction
diff --git a/sidebars.js b/sidebars.js
index 83fcd52..c45a91e 100644
--- a/sidebars.js
+++ b/sidebars.js
@@ -31,6 +31,7 @@ module.exports = {
           "api/utils/suspend",
           "api/utils/suspended",
           "api/utils/switchMapSuspended",
+          "api/utils/toKeySet",
           {
             type: "category",
             label: "Deprecated",